Recent findings indicate that the immune and neuroendocrine systems interact and modulate one another functionally. The mechanism for this seems to be that the 2 systems share a set of receptors and ligands (hormones). Cells of the immune system are able to synthesize neuroendocrine peptide hormones which are biologically active and produced in physiologically significant quantities. Furthermore, leukocytes possess functional receptors for these same neuroendocrine hormones which will specifically modulate immune responses. The structural and functional evidence for these interactions is reviewed and discussed in the context of a bidirectional regulatory circuit between the immune and neuroendocrine systems. © 1985.